Screw.



.P. L. ROBERTSON.

SCREW. APPLICATION FILED OCT. 24, 1907.

Patentd Nov. 8; 1910.

' UNITED STATES PATENT OFFICE.

PETER LYMBURNER ROBERT SON, OF HAMILTON, ONTARIO, CANADA.

SCREW.

tion of the same.

vThis invention relates to screws, the heads of which have axial driving recesses or cavi-- ties instead of transverse slots unched therein and the lnv'ention consists in a recess or cavity extending into the screw head,

the outer portion of the recess or cavity being prismatic and the inner portion thereof being pyramidal, the apex of the pyramid being in the axial line of the prism and of the screw. For full comprehension, however of my invention reference must be had to the accompanying drawings forming a part of this specification, in which similar reference characters indicate the same parts and wherein- Figure 1 is an enlarged axial sectional view of a screw embodying my invention; Fig. 2 is a top view of the same; Fig. 3 is a perspective view thereof at reduced scale;

' and Fig. 4 illustrates the screw driver point I prefer to employ in connection with my invention. 1

The body A of the screw and the'exterior I of its head B are of usual construction and in the head is punched my improved axial recess C. The recess is. square or prismatic in cross section throughout its depth fi the outer portions a of the four walls or sides beingflat and equal in area and the inner portions 1) being also equal in area and sharply converging in pyramidal form-to an axial point, the angle of convergence being approximately forty five degrees to the axial line of the screw. The outer edges of the recess are flattened or beveled as at e Specifi cation of Letters Patent. Application filed October 24, 1907. Serial No. 399,010.

and in the event of the screw being struck with a hammer in starting it, as is the practice, there will be no danger of the edge'being turned in to narrow the opening. The advantages of forming the cavity with walls presenting the outerprismatic or square portion and the inner pyramidal'portion converging at anangle of forty five (45) degrees and also square in cross section, are: first, that during thepunching of the cavity there is an equal distribution of metal in four directions laterally from an axial starting point instead-of a compression altogether in advance of the punch: secondly, in driving the screw a. combined effective turning power and centralizin 9f fect is possible as well as an equaldistribution of strain, the centralizing effect correcting any tendency of. the screw todeflectfrom. direct alinement with the driver and the vertical, horizontal and inclined corners, presented by the particular form of the walls, increasing the bearing surfaces for the driver.

What I claim is as follows:

A screw having a recess extending into its Patented Nov. 8, 1910..

matic to present four flat sides and the inner portion being pyramidal to also present four flat sides in continuation of and atangles of 45 degrees to the sides of the outer portion and the apex of the'pyramid being in the axial line of the prism and of the screw.

liamilton, Ontario, Canada, October 14th 190 7 PETER LYMBURNER ROBERTSON. 

